Waveguiding epitaxial LiNbO3 films

ABSTRACT

A process is described for making optical circuits on LiTaO 3  substrates. The process involves putting down an epitaxial layer of LiNbO 3  on the LiTaO 3  substrates. Growth is preferably carried out on certain planes of the LiTaO 3 . These optical circuits are unique in that they are smooth, uniform in thickness and have a refractive index significantly larger than that of the substrate. This is advantageous in optical circuitry, since the optical modes in the circuits are quite distinct and can be coupled separately using such light sources as lasers and light emitting diodes.

BACKGROUND OF THE INVENTION

Extensive work is being carried out in the development of materialssuitable for optical waveguiding devices. Principal problems in thefabrication of optical waveguiding devices are the need for high opticalquality including freedom from defects and smooth change from substrateto optical film. It is also desirable for many applications to have asharp and large change in refractive index from substrate to opticalfilm. This produces widely separated modes which are easy to couple intoand out of integrated optical circuits.

Particularly attractive, principally because of high index ofrefraction, is thin films of LiNbO₃. Growth techniques reported in theliterature include diffusion, vapor deposition and liquid phaseepitaxial processes. The compound LiTaO₃ is commonly used as thesubstrate because it is isostructural with LiNbO₃ which permitsepitaxial growth of thin films. None of the earlier methods are,however, completely satisfactory. For example, diffusion processproduces a layer with relatively small refractive index differencebetween film and substrate (usually less than 1%), although it does havethe advantage of leaving the substrate surface in its original highlypolished state, which greatly facilitates device fabrication. Liquidphase epitaxial growth through flux is a useful process, but the fluxusually introduces additional loss to the circuit and thus undesirablefor some applications.

The melt phase process produces a large refractive index change betweensubstrate and film but the nature of the high temperature melt phasereaction sometimes introduces optical imperfections. For example,Miyazawa (Appl. Phys. Lett. 23, 198 (1973)) describes melt phaseepitaxial growth of LiNbO₃ on LiTaO₃ substrates. The films were grown onthe C and A planes. It is highly desirable to have a process for makingoptical waveguiding circuits which yield both high index changes andleave the substrate surface in its original polished condition.

SUMMARY OF THE INVENTION

The invention is a process for making optical circuits on LiTaO₃substrates in which an epitaxial layer of LiNbO₃ is grown on the LiTaO₃substrates. Only certain specific crystallographic planes of LiTaO₃ areused for growth of LiNbO₃. These crystallographic planes which arecrystallographically equivalent in the LiTaO₃ structure are the 10.2,11.2, and 01.2 planes. Certain other crystallographic planes which arenearly equivalent to these planes may also be used. They are the 10.2,01.2 and 11.2 planes. Growth is carried out by first putting powderedLiNbO₃ on the LiTaO₃ substrate and heating to above the meltingtemperature of LiNbO₃ and slow cooling. The LiNbO₃ powder may be appliedto the substrate in two ways: First, LiNbO₃ powder is suspended in anorganic vehicle and then painted or sprayed on the substrate. It ispreferred that the organic vehicle have a reasonable viscosity,generally above 800 centipoise at room temperature (20° C). Thesubstrate with painted suspension is slowly heated to a temperaturebetween the melting temperature of LiNbO₃ and LiTaO₃ and then cooled atbetween 10° and 50° C per hour to below the melting point of LiNbO₃.Film thickness can be controlled by varying the thickness of the appliedliquid suspension. In another procedure, film thickness is controlled byapplication of pressure between the molten LiNbO₃ and the LiTaO₃substrate. A thin sheet of platinum is used for this purpose and ispeeled away after the growth process is completed. Films grown inaccordance with the above procedures have high optical quality and rapidindex change from substrate to grown film. Experiments show that theyform excellent optical waveguides and are particularly useful forforming electrooptical devices such as light beam scanners, polarizationswitches, etc.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows in diagrammatic form an optical device utilizing theepitaxial layer of LiNbO₃ on LiTaO₃ ;

FIG. 2 shows in diagrammatic form an electrooptic device utilizing anepitaxial layer of LiNbO₃ on a LiTaO₃ substrate;

FIG. 3 shows in diagrammatic form a light scanning device utilizing anepitaxial layer of LiNbO₃ on a LiTaO₃ substrate.

DETAILED DESCRIPTION

The invention is a process for making optical circuits on LiTaO₃substrates in which an epitaxial layer of LiNbO₃ is grown on thesubstrate. The LiTaO₃ substrate is oriented crystallographically sogrowth takes place on a specific crystallographic plane. Thiscrystallographic plane is the 10.2 plane or equivalent plane (11.2 and10.2). Equally good results are obtainable from closely related planes,namely the 10.2, 01.2 and 11.2 crystallographic planes. It has beenfound that LiNbO₃ films grown on these crystallographic planes areunusually uniform and smooth which makes them particularly suitable foroptical applications. It also permits use of these films in opticalapplications with little or no polishing which substantially simplifiesand reduces the cost of commercial production.

The particular procedure for growing the LiNbO₃ epitaxial layer is alsoof importance. A melt phase procedure is used. This procedure produces asubstantial refractive index change from substrate to film which permitsgreater resolution of individual optical waveguide modes. First, apowder of the LiNbO₃ is put down on the LiTaO₃ substrate. Particle sizeof the LiNbO₃ powder has a profound effect on the results of theepitaxial growth process. Small size, preferably less than 50 microns ishighly desirable. Such a powder may be made by grinding a siftingthrough a mesh (generally a 300 mesh). Uniform particle size is alsodesirable and this may be accomplished by sifting through another mesh(say 400) to remove smaller particles. This yields a powder withparticle size approximately in the range from 25-50 microns. Smallersizes are desirable but more difficult to produce.

The substrate surface may be polished for best results although suchpolishing is often not necessary. To insure good control of the filmthickness throughout the epitaxial layer, the LiNbO₃ powder may besuspended in a type of substance which should evaporate or decompose onheating. This suspension can then be sprayed or painted onto the LiTaO₃substrate. Reasonably high viscosity is preferred, say 800 or even 2000centipoise at 20° C to insure that the powder remains approximatelyuniformly distributed in the organic vehicle. Typically, a lacquersuspension is used. After applying the suspension to the substrate thesample is subsequently heated to a temperature above the meltingtemperature of LiNbO₃ but below the melting temperature of LiTaO₃. Onheating, the organic vehicle evaporates or decomposes leaving a uniformdistribution of LiNbO₃ powder on the substrate. On reaching the meltingpoint of the LiNbO₃ the powder melts. The substrate with molten LiNbO₃is maintained at a temperature between the melting temperature of LiNbO₃and LiTaO₃ (1260°-1320° C) for some time (typically 1 minute to 2 hours,depending on desired amount of solid solution). On subsequent cooling,the epitaxial crystalline layer forms. This procedure of lacquersuspension application and heat cycling may be repeated to adjust thethickness of the epitaxial layer.

Film thickness may be controlled in another way which does not involveuse of a suspension of LiNbO₃ powder. In this procedure LiNbO₃ powder isput down on the LiTaO₃ substrate and a thin sheet of inert material suchas platinum is put down on top of the LiNbO₃ powder. The film thicknessis controlled by the application of various pressures or weights on theplatinum sheet. Typical weights are 1-20 gms over a cm² area. Mostuseful thicknesses are obtained with a 5-15 gm/cm² pressure. The growthprocess is carried out as above by heating the substrate with powder toa temperature between the melting point of LiNbO₃ and LiTaO₃. Theplatinum sheet may be peeled away after completion of the growthprocess. Generally, film thicknesses are typically from 1 to 20micrometers.

Because the crystal structure of the LiTaO₃ substrate and LiNbO₃ filmare closely related, the two materials form good epitaxial layers. Also,it is believed that some solid solution occurs between the two materialswhich causes the lattice parameter to be graded at the interface. Suchcondition leads to excellent epitaxial layers with a minimum of strainbetween layer and substrate.

In the preparation of the LiNbO₃ powder certain dopants may be added toobtain desired optical properties. A particular example is the use ofrare earth ions, such as neodynium to produce laser action. Also, slightvariations in the heat cycling procedure can be used to alter thecomposition profile of the LiNbO₃ film. For example, it is believed thatthe films formed on the substrate are actually solid solutions of LiNbO₃and LiTaO₃ in which the concentration of LiTaO₃ decreases with distanceaway from the substrate. This composition profile is a function ofgrowth temperature and may be varied by varying the conditions ofgrowth.

A typical procedure is as follows: LiNbO₃ powder having an averageparticle size of approximately 25 to 50 micron units is suspended in acommercial lacquer and then painted or sprayed onto a prepolished LiTaO₃substrate. Particularly good results in terms of smooth films of uniformthickness is obtained if the LiNbO₃ powder has small particle size anduniform particle size. It is preferred that 80 percent of the LiNbO₃powder has particle size between 25 and 50 microns. The substrate withpainted suspension is slowly heated to a temperature between 1260° and1320° C. During the early stages of heating the organic vehicledecomposes or volatilizes usually at about 600° C. This leaves a uniformlayer of LiNbO₃ powder on the substrate. At about 1260° C the powdermelts. The substrate is usually maintained in the temperature range from1260° C to 1320° C for some period of time. The substrate is then slowlycooled at about 10° to 50° per hour yielding a LiNbO₃ single crystallayer epitaxially joined to the substrate of LiTaO₃. A cooling rate of15° to 30° C is preferred because it yields epitaxial layers of highoptical quality. Generally, the slow cooling need only extend to about600° C.

The Figures show various devices which can be made using the inventiveprocess. FIG. 1 shows an integrated circuit 10 which is useful inprocessing laser radiation. This epitaxial device is made up of asubstrate of single crystal lithium tantalate 12 with a layer ofepitaxial lithium niobate 11. Laser radiation 15 originates from a laser18 and is coupled into the epitaxial layer 11 by means of a prism 13.The laser radiation inside the epitaxial layer passes through anepitaxially active region 17 which processes the laser radiation. Theprocessed laser radiation 15 is coupled out of the epitaxial layer bymeans of a prism 14 and then into the utilization apparatus 19.

FIG. 2 illustrates a different type of epitaxial device 20. Again themain part of the epitaxial device consists of a lithium tantalatesubstrate 22 with an epitaxial layer of lithium niobate 21. Laserradiation 25 which originates from a laser 28 is coupled into theepitaxial layer by means of a grating coupler 23. Electrodes 26 and 27,generally made of conducting electrode material such as copper, are usedto alter the electrooptic properties of the layer. Particularlyillustrative is the use of these electrodes to couple TF polarizationmodes into TM. A polarizer 24 may be used to remove one of the modes.

FIG. 3 shows a light beam scanning and reflection device 30 with lightbeam 31 and electrodes 32. The device is also equipped with a groundplane 33. On inducing a negative index change as shown at 34, the lightbeam is bent in one direction as shown at 35. On electrically inducing apositive index change 36, the light beam is bent in the oppositedirection as at 37.

What is claimed is:
 1. A process for producing epitaxial crystallinefilms of LiNbO₃ on LiTaO₃ substrates in which the films are grown on acrystallographic plane of LiTaO₃ selected from the group consisting ofthe 10.2, 11.2, 01.2, 10.2, 01.2 and 11.2 planes comprising the stepsof:a. preparing the LiNbO₃ in the form of a powder with particle sizeless than 50 microns, b. preparing a suspension of LiNbO₃ in a viscousliquid organic medium which evaporates or decomposes on heating, c.spreading some of the suspension of LiNbO₃ on the LiTaO₃ substrate, d.heating the LiTaO₃ substrate to a temperature between 1260° and 1320° C,e. maintaining the temperature of the LiTaO₃ substrate between 1260° and1320° C for from 1 minute to 2 hours, f. cooling the LiTaO₃ substrate ata rate of 10°-50° C per hour to below 600° C.
 2. The process of claim 1in which the viscous liquid organic medium has a viscosity of at last800.
 3. The process of claim 2 in which viscous organic medium is alacquer.
 4. The process of claim 1 in which the particle size is between25 and 50 microns.
 5. The process of claim 1 in which the cooling isbetween 15° and 30° C per hour.
 6. A process for producing epitaxialcrystalline films of LiNbO₃ on LiTaO₃ substrates in which the films aregrown on a crystallographic plane of LiTaO₃ selected from the groupconsisting of the 10.2, 11.2, 01.2, 10.2, 01.2 and 11.2 planescomprising the steps of:a. preparing the LiNbO₃ in the form of a powderwith particle size less than 50 microns, b. distributing some LiNbO₃powder on the surface of the LiTaO₃ substrate, c. placing a platinumfoil on top of the LiNbO₃ powder on the LiTaO₃ substrate, d. applying apressure of 1-20 gms/cm² to the platinum sheet, e. maintaining thetemperature of the LiTaO₃ substrate between 1260° and 1320° C for from 1minute to 2 hours, f. cooling the LiTaO₃ substrate at a rate of 10°-50°C per hour to below 600° C and then to room temperature, removing theplatinum foil.
 7. The process of claim 1 in which the particle size isbetween 25 and 50 microns.
 8. The process of claim 1 in which thecooling is between 15° and 30° C per hour.